Interpretive Summary: Gloss is an attribute that causes the surface of an object to have a shiny or lustrous appearance. Gloss is an important physical aspect of appearance for many fresh fruits and vegetables. Although the gloss of some fruits, particularly apples, is not necessarily correlated to their quality, consumers often perceive the lack of gloss in apples to be of low quality. Hence, gloss is added artificially, usually by applying waxes, to enhance product appearance. Instrumental gloss measurement is thus needed in evaluating the effectiveness of different waxing procedures and providing objective judgment of fruit appearance. Most commercial glossmeters are designed for products with uniform, flat surfaces, and they may not be suitable for fruits and vegetables because of their curved surface and variable sizes. In this research, we evaluated a commercial glossmeter for measuring the gloss of apples, nectarines, plums, and tomatoes. Experimental tests showed that considerable skill and care was needed in using the commercial glossmeter to measure gloss on fruits, and results obtained could be variable and inconsistent. To provide more objective, accurate gloss measurement, we designed, built and tested an automated laboratory gloss measurement prototype for apples and other horticultural products with curved surface. The prototype captured images from the sample in two perpendicular directions and then automatically located appropriate locations on the sample for gloss measurement. The prototype had a nonlinear relationship with standard gloss measurement. Results obtained from apples showed that the prototype had a repeatability value of equal to or less than 5% for 50% of the measurements and 16% for 90% of the measurements. The prototype is useful for objective measurement of the gloss of apples and other horticultural products. It provides researchers and fruit packers a means for assessing and improving the appearance of fruit.

Technical Abstract:
Gloss measurements are used to evaluate the quality and appearance of a product, especially in the cases where aesthetic appearance is of importance. Most commercial glossmeters are designed for measuring products of flat surface and therefore are not suitable for food products because of their uneven, curved surface. A new generation of commercial glossmeters is recently available for gloss measurement on confined areas, curved surfaces, and surfaces that are not uniform. However, none of them has been studied for fresh agricultural produce such as apples and no standard measurement procedures have been established. This article provides a brief review of gloss measurement principles and methods for fresh fruits and vegetables. It reports on the gloss measurement of apples and other fresh products using a commercial glossmeter and a specially designed spectrometer-based gloss measurement prototype integrated with imaging and automatic sample positioning capabilities. Results showed that the prototype had a nonlinear relationship with standard gloss measurements. The prototype was able to measure the gloss of apples with the repeatability for 90% of the measurements being better than 16%, calculated as the ratio of the difference between maximum and minimum gloss values to the mean value in percent. Further research is needed to simplify the imaging/mechanical configuration and improve the coordinate calculation algorithm in order to achieve more accurate, repeatable gloss measurements.